The fluidized bed technology in olefin polymerization reactors used today can be adjusted to produce a wide variety of products. This is particularly true for polyethylene manufacture. It is not unusual to demand that one system produce resins that can be used in injection molded, blow molded, roto-molded products, wire coating, piping and tubing, and films. Fluidized bed technology can be used to make a wide variety of polyolefin products, e.g., homopolymers and copolymers of polyethylene, polypropylene, C.sub.4 -C.sub.12 alpha olefins; ethylene-propylene-diene monomer (EPDM), polybutadiene, polyisoprene, and other rubbers. Generally, the polymer products made by a given reactor system use the same reactants but in different ratios and at different temperatures. Each of these polymer products can be made with a number of different resin properties, or grades. Each grade of polymer product has a narrow limit on its properties, e.g., density and melt index.
The length of time a reactor is used to make a particular type of polymer depends on the market demand for the product. Some products can be run for weeks without change. Other products are made for much shorter periods of time. Unfortunately, industrial reactors require time to adjust to the new conditions (e.g., temperature, reactant pressures, and reactant ratios) and produce material in the interim that is constantly changing but not within the properties (e.g., melt index and density) of either the old product or the new one. New products cannot be made instantaneously and require a quantifiable period of transiency in becoming adjusted to the new, desired conditions. Similarly, reactors operating at fixed conditions, i.e., at "steady state", can experience fluctuations that can result in the production of "offgrade" material. This offgrade material that; represents an economic loss and is desirably minimized.
Generally, industrial control systems for gas phase, fluidized bed polymerization reactors are designed to permit the operators to control the reactor by allowing the operators to select a desired melt point index and density. Correlations of these properties are usually well known by the operators and those in the art for the particular reactor design and catalyst used.
The prior art has devised a number of methods to reduce the transient, offgrade material. These methods typically involve some combination of adjusting the automatic flow/ratio controllers to a new value either at or above the ultimately desired value ("dial-in transition" and "overshoot")), removing the reactant gas entirely ("inventory blow down"), reducing the level of the catalyst ("low bed"), and adding a nonreactive gas ("nitrogen addition").
DE 4,241,530 describes using a kill gas to stop a polymerization reaction, blowing the gas inventory for that reaction out of the reactor, and rebuilding a new gas inventory for a new product. This method reduces transition material. The costs associated with throwing away the old gas inventory and rebuilding a new inventory are too high for commercial transitions between closely related grades. Thus, most transitions between grades of the same material are performed by adjusting the reaction conditions.
McAuley et al. ("Optimal Grade Transitions in a Gas Phase Polyethylene Reactor", AIChE J., Vol. 38, No. 10: 1992, pp. 1564-1576) discloses three manual, labor-intensive transition strategies for gas phase polyethylene reactors. The first is an adjustment to the controls to overshoot the melt index and density values. The hydrogen feed and co-monomer: feeds are increased to meet the designated properties. The actual desired setpoint values are directed when the sensors indicate that the desired product is being produced. The second is an increase in temperature and manipulation of the slow vent to move the melt index of the produced product. The third is a drop in the catalyst level of the lower bed while keeping the bed resin residence time at a constant to reduce offgrade production.
Debling, et al., "Dynamic Modeling of Product Grade Transitions for Olefin Polymerization Processes", AIChE J., vol. 40, no. 3: 1994, pp.506-520) compares transition performance of different types of polyethylene reactors. The article discloses seven separate manual, labor intensive transition strategies: (1)dialing in the final aim transition; (2) gas inventory blow down and simple dial-in transition; (3) low bed and simple dial-in transition; (4) gas inventory blow down and overshoot of melt index and density transition; (5) low bed, gas inventory blow down, land overshoot transition; (6) low bed and overshoot transition; and (7) gas inventory blow down, overshoot, and nitrogen addition transition.
Despite these wide variety of available schemes, there is a continuing need and desire to reduce the amount of offgrade material produced during transition to a new product grade or during steady state manufacture.